It is well known that coatings play a useful role in the manufacture of a wide variety of useful articles. Until recently, nearly all coatings were formulated and applied by employment of an organic solvent, which often comprised a major portion of the total formulated coating. After the coating is applied to the article to be coated, the organic solvent is evaporated leaving the dried coating on the article to serve its decorative or functional purpose. This coating system has met with increasing disfavor as the cost of energy needed to evaporate the solvent at the rate required by industry increased, as the price of the solvent increased, and as the deleterious environmental effects of the evaporated solvent became better understood. In addition, governmental regulations have placed ever increasing restrictions on the amounts and types of solvents or organic volatiles permitted to escape into the atmosphere from coatings compositions. Systems aimed at solvent recovery to reduce pollution and conserve solvent have generally proven to be energy intensive and expensive.
Considerable efforts have been expended by those skilled in the art to develop coating compositions having a minimal amount of volatile organic components and this has led to development of powder coatings, radiation-curable coatings, water borne coatings and high solids coatings. In these recent developments, the amounts of organic solvents present are minimal and consequently there is little or no atmospheric pollution.
Among the new coating systems, radiation-curable coatings, usually cured with ultraviolet light or electron beam radiation, offer a variety of advantages. They require only minimal energy to effect cure--change from liquid to solid state--they do not contain solvents, and thus do not cause deleterious effects to the environment, and they are cost effective, since effectively all of the applied liquid is converted to a solid coating.
An important disadvantage of photocurable systems is the frequent requirement that the curing process be conducted in an inert atmosphere because of the inhibiting effect of oxygen.
Responding to such problems, those skilled in the art have devised photocurable coatings which cure through a mechanism termed cationic polymerization. In these systems, the starting materials are mixed with catalysts which form acids when exposed to ultraviolet light; the starting materials are therefore polymerized via cationic catalysis.
Epoxy resins, linear vinyl ethers, and cyclic vinyl ethers have been shown to be suitable starting materials for photocure via cationic polymerization, as disclosed in, for example, U.S. Pat. No. 3,794,576; the publication "New Monomers for Cationic UV-Curing", Conference Proceedings, Radiation Curing VI, pages 4-28, Sep. 20-23, 1982; and British publication No. GB 2,073,760A.
However, a photocurable coating composition that can be cured by cationic polymerization and having increased toughness over compositions as are currently known would be highly desirable.